One-component and two-component, moisture-curable, isocyanate-based coating formulations are well-known and are widely used. Generally, the one-component compositions are based on isocyanate-terminated prepolymers and have a viscosity sufficiently high as to make application to substrates, especially by such techniques as spraying, readily achievable only by dilution with a solvent or solvent mixture to solids levels of 50% or less. The two-component isocyanate-based coating compositions generally require less solvent than one-component isocyanate-based coating compositions, but still suffer from a high solvent demand by acrylic polyester polyols typically used in these coating compositions in order to achieve a suitable application viscosity. However, due at least in part to the introduction of the Clean Air Act of 1970, coatings formulators are under pressure to reduce volatile organic compounds ("VOC"), which translates to the solvent demand of the coating composition, to as low a level as technological advances will allow. Previous attempts to reduce the amount of acrylic polyester polyol, and hence the solvent demand, have led to a loss in the desired properties of the coating. Therefore, it would be highly desirable to provide new coating compositions, which are inherently low in viscosity and contain minimal or no solvent, but which exhibit excellent properties associated with moisture-curable, isocyanate-based coatings.
U.S. Pat. No. 5,144,031 discloses both heat-crosslinkable, one-component blocked-isocyanate compositions and two-component coating compositions which are said to harden at room temperature or at a slightly elevated temperature. Column 7, lines 46-52 of the '031 patent discloses the preparation of two-component coating compositions containing (a) a polyisocyanate prepolymer containing monohydric alcohol-containing ester groups incorporated therein and (b) an isocyanate-reactive polyhydroxyl compound. The ratio of NCO groups on the polyisocyanate to isocyanate-reactive groups is disclosed at column 8, lines 13-17 of the '031 patent calculated as being between 0.33 to 1.25 per isocyanate-reactive group.
As yet another illustration, U.S. Pat. No. 4,355,138 discloses the modification of polyisocyanates having an average functionality of greater than about 2.0 in order to make them compatible with apolar solvents, with the disclosed end use being reaction with alkyd resins for auto refinishing. The modifications are made by reacting a polyisocyanate having only aliphatically or cycloaliphatically bound isocyanate groups with (a) a compatibility modifier which is an aliphatic or cycloaliphatic monohydroxy alcohol having at least 8 carbon atoms and (b) a solubility modifier which is a monohydroxy compound containing one or more polarity inducing groups comprising a member selected from the group consisting of ester groups, ether groups, aromatic rings or mixtures thereof. In a preferred embodiment of the invention of the '138 patent, the solubility modifier and the compatibility modifier are combined into one adduct for reaction with a biuret-containing polyisocyanate.
Unfortuantely, the VOC level of the compositions of the '487 and '031 patents is not as low as might be desired, and the disclosure of the '138 patent is limited to providing enhanced compatibility and solubility in apolar solvents. Accordingly, new, lower VOC compositions which do not employ the prepolymer intermediate of the '031 patent or the polyurethane intermediate of the '487 patent, and which instead employs simpler formulations, exhibiting utility in the presence or absence of a wide variety of solvents, would be highly desired by the two-component coatings community. The present invention provides one such simplified composition.